Flexible diffusers are conventionally used to support aerobic biological processes in wastewater treatment plants. A flexible diffuser typically comprises a disc-, tube-, or strip-shaped membrane that is constructed of rubber or other similar materials, which is punctured to provide a number of perforations in the form of holes or slits. In operation, pressurized air is sent through these perforations to create a plume of small bubbles. The bubbles, in turn, rise through the wastewater and, in doing so, provide the surrounding wastewater with the oxygen needed to sustain the desired biological processes occurring therein.
FIG. 1 shows a front perspective view of a partially cutaway fine bubble diffuser unit 100 that might be used in a wastewater treatment facility. Wastewater treatment with such units is described in, as just one example, F. L. Burton, Wastewater Engineering (McGraw-Hill College, 2002), which is hereby incorporated by reference herein. In the diffuser unit, a flexible diffuser membrane 110 sits atop a diffuser body 120. The diffuser body comprises a threaded connector 130, an air inlet orifice 140, and a receiving surface 150 for coupling to a retainer ring 160. The retainer ring holds the flexible diffuser membrane against the diffuser body. When gas is applied to the flexible diffuser membrane through the air inlet orifice, the gas pressure expands the membrane away from the diffuser body and causes the membrane's perforations to open so that the gas discharges through them in the form of fine bubbles. When the gas pressure is relieved, the membrane collapses on the diffuser body to close the perforations and prevent the liquid from entering the diffuser body in the opposite direction. Generally, a flexible diffuser membrane configured in this way produces bubbles smaller than five millimeters in diameter. The resultant large ratio of surface area to volume in these bubbles promotes efficient oxygen mass transfer between the bubbles and the surrounding wastewater.
Although flexible diffuser membranes are advantageous in many respects and have achieved widespread acceptance in a variety of gas diffusion applications, they are not wholly free of problems. In a wastewater treatment application, materials in the wastewater can become deposited on and build up on the membrane to clog or partially clog the perforations. For example, fats, greases, and other organic substances which are commonly found in wastewater can adhere to the membrane causing fouling. Calcium and calcium compounds such as calcium carbonate and calcium sulfate as well as other inorganic substances are especially problematic when they precipitate and build up on the diffuser membrane causing scaling. Such membrane contamination reduces the efficiency of the aeration system by requiring that the air source work harder to overcome the added flow resistance (i.e., head loss) at the membrane-wastewater interfaces. In addition, efficiency is further impacted because the bubbles typically become larger and the plumes of bubbles become less spatially uniform.
Several attempts have been made to mitigate these problematic aspects of flexible diffuser membranes. U.S. Patent Publication Nos. 2007/0001323 to Kang et al., and 2007/0128394 to Frankel et al., as well as U.S. patent application Ser. No. 12/221,809 to Frankel et al (all three hereby incorporated reference herein), for example, teach the use of fluoroelastomer- and polytetrafluoroethylene-containing coatings along with fluorine-based surface conversions which help to slow the contamination of diffuser membranes. However, while these efforts have had some success at increasing the useful life of diffuser membranes in wastewater treatment applications, even greater improvements to the aeration efficiency and contamination resistance of diffuser membranes remain desirable.